Automatic arc-welding machines and arc-welding robots are used for unmanned, labor-saving automatic welding operations in, for example, an automatic welding line in an automobile assembly shop The welding torch of such,.automatic arc-welding machines and arc-welding robots is provided with an arc sensor for continuously detecting welding spots, to thereby achieve an accurate and precision arc-welding. The arc sensor has a light emitting element, a light receiving element and an objective lens, which are contained in a container disposed behind a transparent objective window made of a transparent material, projects a beam of light for the detection of welding spots through the transparent objective window onto a work to be welded, receives a beam of the light reflected by the work via the transparent objective window, protects the welding spot detecting system by the transparent objective window from an ingress of external foreign substances such as sputtered particles and soot generated during arc welding, and prevents adverse effects of the arc light on the transmission of the detecting light beam. The conventional arc sensor incorporates various devices including a transparent objective window made of a transparent resin or a heat-resistant glass, so that the attaching of sputtered particles to the surface of the transparent objective window is prevented, and employs a laser beam emitted as a detecting light by a semiconductor laser beam emitter and having a frequency within a specific frequency band, to thereby permit the detecting light to be transmitted through the arc light.
Nevertheless, the conventional arc sensor of the above-mentioned construction has a drawback in that the accuracy and sensitivity of the detection are lowered due to a loss of the transparency of the transparent objective window resulting from not only the scorching of the transparent objective window by sputtered particles scattered from the surface of the work during arc welding but also a smoking of the window by soot from the arc-welding flux, and thus the transparent objective window must be often periodically cleaned.
Further, the periodical cleaning of the transparent objective window requires manual labor to implement such a cleaning operation, and therefore, reduces the labor-saving effect of the unmanned, labor-saving assembly line.